Abundances were recoded in term of presence-absence. Geostatistical interpolation : the spatial variation of biological data were analysed using GENSTAT (GENSTAT 7 Committee, 2004), which is a GENeral STATistics package including the main geostatistical tools. It computes experimentala variograms, fits these with various authorised mathematical models and uses them to calculate kriged estimates on a fine regular grid (of latitudes and longitudes). The grid of points was imported into ArcMap and interpolated with the Spatial Analyst extension in order to create a continuous raster of 1 km² resolution. The resulting maps illustrate the spatial distributions and the variations over time for biological data studied in CHARM's area. For legends of maps, approximates of the 5th and the 95th quantiles were used for the minimales and maximales values respectively.

The survey extends from the Eastern English Channel to the south of the North Sea, which corresponds to ICES divisions VIId and IVc. The study area is divided into rectangles of 15° latitude and 15° longitude using a systematic sampling strategy. The sampling gear is a high opening bottom trawl well adapted for catching demersal species, with a 10 mm mesh size (side knot) for catching juveniles. This sampling gear is polyvalent and is well adapted to the varying seabed types encountered in the

study area. The fishing hauls are chosen using professional fishing plans or found by prospecting. The fishing method is standardised: sampling stations have been each year at similar locations and identical sampling gear is used. At each sampling station, all the fish species are sorted, weighed, counted and measured.